1. Technical Field
This disclosure generally relates to monopile foundations commonly employed to support offshore structures and particularly wind turbine generators positioned offshore. In particular, the disclosed embodiment relates to the grouted cylindrical connection between the transition section supporting the wind turbine tower and the single foundation element, or monopile, which transmits the forces and moments from the transition section into the soil strata below the seafloor.
2. Description of the Related Art
Conventional transition—monopile foundations employ a large diameter cylindrical shaped monopile typically driven into the seafloor having adequate length to extend above the sea surface. The larger diameter transition section, including appurtenances for access, egress, maintenance, cable support and equipment support along with a mating flange for connection to the wind turbine tower flange, is lifted and lowered over the monopile until temporarily supported on landing points inside the transition. The landing points are equipped with a hydraulic leveling system or other means to adjust the transition to achieve vertical tolerance when the driven monopile is out of vertical tolerance. After correcting vertically to within acceptable tolerance, the transition section is connected to the monopole by filling the annulus space between the transition and monopile with grout. The forces and moments acting on the transition are transmitted through the grout primarily by shear into the monopile. In order to increase the adhesion between the inner wall of the transition section and the outer wall of the monopiles, shear keys (typically concentric rows of weld beads or rolled flat bars) are applied to both surfaces of the annulus. However, recent long term experience has identified a failure mechanism that requires a departure from the conventional grouted cylindrical connection. The common solution to mitigate grout failure is to add conical sections to the transition section and monopile. However, this solution adds significant cost and the supply of large diameter conical sections has limitations. Further, the mounting of offshore wind turbine towers to transition sections with a cylindrical section require a grouted connection with adequate strength to transmit the forces and moments to the monopile and having excellent fatigue resistance characteristics due to the extensive cyclic loading induced by the wind and waves.